The combination of a gas chromatograph (GC) with a mass spectrometer (MS), possibly associated with a computer is one of the most powerful tools utilised in analytic chemistry and is, in particular, applied to analysis of gases.
Significant progress has been made in this technique, especially through the development of capillary columns in fused silica with small diameter and flow rate and the appearance of new mass spectrometers with high pumping capacity.
Thus, the microchromatograph (μCG) is an apparatus making it possible to carry out high resolution and high performance analysis of complex mixtures rapidly, for example in less than three minutes.
The detector is a non-destructive microkatharometer, which explains why it is of interest to couple the microchromatograph (μCG) with a mass spectrometer which adds the possibility of reliable identification of each component separated by the chromatographic column.
A significant problem of devices combining a chromatograph and a spectrometer relates to the choice of the coupling system forming the interface between the two apparatuses.
The problem of coupling varies according to the chromatographic device used and the pumping capacity of the spectrometer. The preceding generation of classic katharometers imposed the use of full columns, of large diameter, for example several millimeters, and a high vector gas flow, for example several tens of milliliters per minute. Coupling with a mass spectrometer then required a flux separator and differential pumping, always difficult to control [1].
The katharometer microdetector (microkatharometer) coupled to capillary columns with low flow rate (1 to 2 ml/min.) is now perfectly compatible with the pumping capacities of a state of the art mass spectrometer.
Direct on-line coupling without any separator is thus possible, nonetheless on the condition that it is ensured that a permanent load loss of one bar is maintained between the microdetector and the mass spectrometer source: this is the function ensured by a specially dimensioned capillary tube.
Assuming that this condition is satisfied, it is possible in practice to carry out the linking to the microdetector in two ways: either sealed, or open.
The sealed coupling is shown in FIG. 1. In this coupling the flow from the detector 1 arrives directly in the source 2 through the intermediary of the capillary tube 3 and the interface 4.
This coupling risks perturbing the operation of the microkatharometer detector, subject to a variation of the spectrometer pumping capacities or modifications of analysis conditions, in particular the head pressure and/or temperature of the column.
In other words, the sealed coupling has the advantage of a yield of 100%, but to the detriment of optimum operation of either the microkatharometer, which may be in depression, or the spectrometer, whose vacuum may become defective through saturation of its pumping capacities.
On the other hand, the connection or open coupling 7, as shown in FIG. 2, has the advantage of preserving the optimum operational conditions of the two detectors, and the retention times are identical to those obtained in classic chromatography [2]. This mounting usually operates according to the following principle: the load loss of the transfer line is imposed, which is one bar, the diameter and the length of the capillary 3 being chosen in such a way that a vector gas flow close to the tolerable maximum for the spectrometer (source 2) crosses through the transfer line, and this flow rate is as close as possible to the flow leaving the micro-detector [3].
The authors advise addition of helium 5 to compensate for a lowered flow rate, this helium protecting the mass spectrometer from air entry. This solution has the advantage of preserving both the mass spectrometer and the katharometer, but it introduces dilution of the solute flow 6, which can be a disadvantage in all cases where trace search is the required aim.
None of the coupling devices known in prior art provides satisfactory coupling between the output from the microchromatograph, that is the katharometer and the mass spectrometer source.
Such a coupling device must ensure several functions and fulfil several requirements, in particular the following:                to ensure operation of the katharometer detector at atmospheric pressure, whatever the analysis requirements relative, in particular, to the head pressure of the column and the temperature of the column.        
This operation is the gauge of optimum sensitivity of the detector and the linearity of the response, in function of the concentration of species;                to register the totality of the flow leaving the micro-detector and consequently to benefit from maximum detection sensitivity of the mass spectrometer;        to preserve the separation of the species already detected, and to transfer them to the spectrometer source, under high vacuum; and        this coupling device must be able to be connected indiscriminately to any one of the modules, numbering four for example, able to equip the microchromatograph. The passage from one micro-detector to another must be very simple and rapid, without disturbing the operation of the spectrometer in any way.        